Ever since antiquity, people have created pictures. In ancient times, pictures were exclusively still pictures generated, for example, by painting, or drawing on a surface. In modern times, photography has provided the ability of creating pictures through technological tools, and cinematography has provided the ability to create moving pictures, first in black and white and, later, in color. More recently, electronic displays, such as computer monitors, TV sets, and projectors, have become the most common devices for displaying moving pictures.
With very few exceptions, electronic displays generate pictures that are perceived the same regardless of the position of the viewer. Indeed, a lot of engineering effort has been devoted to achieving displays with a wide viewing angle and with minimal degradation of the picture even for viewers looking at the display from directions that are very different from optimal. There are, however, situations where it is desirable to have a display that shows different pictures when viewed from different angles. Such displays are known as multi-view displays. For still pictures, techniques have been available for a long time to achieve such as a result, albeit with limited picture quality and with other important limitations.
FIG. 1 depicts a so-called lenticular picture 100 in the prior art. A lenticular picture provides multi-view functionality for still images. It is realized as a picture that a viewer 130 can hold in his/her hand. The lenticular picture comprises a grooved sheet of plastic with a paper backing. The front of the sheet of plastic is shaped such that the grooves form an array 110 of cylindrical lenses, as shown in detailed view 115 in the figure. The paper backing is a print of two or more interleaved images; the print is shown in FIG. 1 as interleaved print 120.
In FIG. 1, the grooves constituting the cylindrical lenses of array 110 are arranged horizontally. As a consequence, the viewer 130 of the lenticular picture can rotate the picture about a horizontal axis 140 in order to see different images. As the lenticular picture is rotated in the direction, for example, of rotation 150, different images become sequentially visible on the viewable surface of the lenticular picture, with each sequential image occupying the entire viewable surface, when visible. While one sequential image is visible, the other sequential images are not visible.
FIG. 2 Illustrates the process of creating interleaved print 120. In this example, the objective is that the final lenticular picture will show two distinct sequential images, one with a large letter “A”, and the other with a large letter “B”.
Each one of the two images is processed by slicing it into a large number of horizontal stripes, and then every other stripe is removed. In FIG. 2, the result of this process for the letter “A” is shown as first image 210-1. The familiar outline of the letter “A” is clearly identifiable even though a large number of white stripes obliterate part of it. The result of the same process for the letter “B” is shown as second image 210-2. Again, the outline of the letter “B” is clearly identifiable.
There is an important difference between the two processed images: in the case of the letter “B”, the stripes that were removed were not the same stripes that were removed when processing the letter “A”; rather, they were the alternate stripes. As a consequence, the two images can be combined with the stripes of one image fitting (interleaving) between the stripes of the other image. The result is shown in FIG. 2 as interleaved image 220, which can be printed on paper to create interleaved print 120 for the lenticular picture.
The functionality of the lenticular picture is based on a phenomenon that can be explained via geometrical optics: at any viewing angle, the cylindrical lenses show only a set of narrow horizontal stripes from the underlying printed image. The set of stripes that is shown depends on the viewing angle, and it changes when the viewing angle changes. By rotating the lenticular picture about horizontal axis 140, viewer 130 can cause the lenticular picture to show different sets of stripes from the underlying print. When the set of stripes being shown falls on top of stripes from the letter “A”, the viewer will see a letter “A”; however, when the viewing angle is changed such that the set of stripes being shown falls on top of stripes form the letter “B”, the viewer will see a letter “B”. In both cases, the other letter is not visible at all because no other stripes are made visible by the cylindrical lenses.
The interleaving process illustrated in FIG. 2 can be implemented for more than two images. For example, a lenticular picture can show three distinct images, each at a different viewing angle. To realize such a lenticular picture, each of the three images is sliced into equal-size stripes, but only every third stripe is retained. The three sets of retained stripes are then combined into a single interleaved print.
For a lenticular picture to operate as planned, the alignment and scaling of the print relative to the lens array must be precise. Of course, the cylindrical lenses must be carefully aligned with the stripes, or else different images might become visible simultaneously in different parts of the picture. Additionally, the spacing of the stripes, relative to the spacing of the cylindrical lenses, must be calculated and implemented with precision. FIG. 1 shows that the viewing angle from the viewer's eyes to the surface of the picture is different in different parts of the picture, and the exact extent of difference depends on the distance between the viewer and the picture. Accordingly, the spacing of the stripes needs to be slightly different from the spacing of the cylindrical lenses, and it depends on the desired viewing distance.
For these reasons, it is difficult to create lenticular pictures with more than a few different images, and it is difficult to achieve image quality comparable to that of conventional pictures. As a result, lenticular pictures have not progressed much beyond novelty items and specialized applications. The problem of achieving the necessary precision alignment remains an important obstacle to wider use of lenticular pictures and other types of multi-view displays.
FIG. 3 depicts a prior-art application for a dual-view lenticular picture. A poster 300 for public viewing is realized as a lenticular picture with horizontal cylindrical lenses. The poster is shown in a public area where both adults and children might be present. The objective of the poster is to show a message intended for adults that might be unsuitable for young children. Because young children are generally shorter than adults, the angle from which they view the poster is different from the angle of view of an adult. This is illustrated in FIG. 3, where child 310 can be observed to have a different view of the poster, compared to adult 320.
The lenticular picture of poster 300 can be adjusted to show one image to individuals taller than a certain height, who can be presumed to be adults, while children, who are shorter, see a different image. For the poster to work correctly and achieve the desired objective, it is necessary to know a number of parameters with good accuracy prior to manufacturing the poster. The needed parameters include, among others, the viewer-height threshold at which the changeover from one image to the other is to occur, the distance between the viewers and the poster, and the height above ground where the poster is going to be installed. Such parameters and others need to be known with a good level of precision, and the installation locale must be such that these parameters do not vary significantly from viewer to viewer. These are significant constraints, and they illustrate the reason why multi-view pictures of this type are not more common.
FIG. 4 depicts another prior-art application of multi-view lenticular pictures. The figure illustrates the principle of operation of a stereoscopic lenticular picture. Seen from above, on the left in the figure, a viewer's head 410 is looking at stereoscopic lenticular picture 400. The viewer's left eye 420 and right eye 430 are depicted in the figure. The viewer's left eye sees the picture from an angle that is slightly different from the angle of view of the right eye.
Unlike the lenticular pictures in the previous figures, the cylindrical lenses in lenticular picture 400 are aligned vertically instead of horizontally and, of course, the multiple images on the interleaved print are interleaved with vertical stripes instead of horizontal stripes. As a consequence, different images become sequentially visible when the viewer moves horizontally relative to the picture.
The left eye and the right eye of the viewer see the lenticular picture 400 from different positions that are shifted horizontally, relative to one another; and the parameters of the lenticular picture can be selected such that the two eyes see different images. The desired stereoscopic effect is achieved when the two different images are the images that the left and right eyes would see when looking at the original subject of the picture.
FIG. 5 shows a lens array in the prior art wherein the individual lenses are spherical lenses instead of cylindrical lenses.
FIG. 6 illustrates the principle of operation of a typical image projector. The illustration applies to old-fashioned movie projectors and slide projectors that project images from film, and it also applies to modern electronic projectors. In all such cases, the image to be projected onto a screen originates as a bright image that emits light, shown in the figure as bright image 610. In the case where film is used for the image, the light comes from a bright light bulb behind the film, and the film acts as a filter that selectively allows the passage of light of different colors and intensities in different portions of the image. A similar technique is used in some modern projectors wherein the filter might be an LCD module or some other type of electronic light filter, instead of film. Alternatively, the bright image might be generated by an array of bright sources such as, for example, light-emitting diodes, or by digital micromirror devices that reflect light from a separate source.
The term “pixel” is widely used in conjunction with images and image processing. It is a contraction of “picture element” and it refers to the smallest image-forming unit of a display. In particular, an image such as bright image 610 is generally composed of a large number of pixels, wherein each pixel emits light in a wide range of directions. Each pixel emits light of a particular color and intensity, such that the collection of all the pixels forms a pattern that is perceived as an image by the human eye.
In a projector, as depicted in FIG. 6, some of the light emitted by each pixel is collected by a lens 620. In the figure, two pixels are highlighted explicitly as pixel 630-1 and 630-2. The figure shows, for example, the light 640-1 emitted by pixel 630-1 and collected by the lens 620. The lens is adjusted such that the light collected from the pixel is focused into a light beam 650-1 with a focal point is on a projection screen some distance away (the screen is not shown explicitly in the figure). When the light beam 650-1 reaches the screen, it produces a bright spot on the screen. The color and brightness of the spot are the same as the color and brightness of pixel 630-1 in the bright image 610. The light 640-2 from pixel 630-2 is also processed by the lens 620 in similar fashion, such that it also produces a bright spot on the screen whose color and brightness are the same as the color and brightness of pixel 630-2. All the pixels of the bright image 610 produce bright spots on the screen in similar fashion. The collection of all the bright spots on the screen forms the projected image.
In a typical projector, very few adjustments are needed for achieving a clear projected image on the screen. Typically, it is only necessary to adjust the focus of the lens for the specific distance of the screen from the projector. Once the focus is adjusted, the image on the screen is equally clear for all viewers. Displays such as conventional television sets and computer monitors require no adjustment at all. In contrast, multi-view displays need extensive adjustments. As noted above, even simple lenticular pictures rely on a precise alignment and precise positioning of the interleaved print relative to the array of cylindrical lenses. More generally, multi-view displays need adjustments that are specific to the viewer's position relative to the display. When multiple viewer positions are possible, the amount of adjustment needed can be substantial, and multi-view displays need to be calibrated in order to achieve the necessary adjustments. It would be advantageous to have multi-view displays wherein the calibration procedure is simple, or automatic, or both.